Subnet Mask : The Quickest Way to Define Subnet Mask

Subnet mask is used to split IP networks into subgroups. Subnet mask defines which bits in the host portion of the IP address can be used to define a subnet. Defining subnet mask is one of the critical and difficult task for the CCNA learners or the network engineer. Some questions (related to subnet) are always set in the CCNA exam. Moreover, many jobs and even in interview questions also include some questions related to subnet. Subnet is always an important era in networking. As it's a bit critical and it's always an important for networking field, many people give much time to learn it. Many courses offer several techniques or many books tell you several methods to learn it. Huge online resources are also available. Are they really useful? I don't know how much they are useful to you. But here, I tried to revel you the quickest techniques to define subnet mask. Just follow some steps and be the hero of subnet mask.

[Note: Visit Subnetting Index or type http://hubpages.com/_ccna/hub/Subnet to your browser address bar to learn a-to-z of subnetting. Many examples are there to make you a smart master of subnetting.]

Before learning the quickest steps, try to memorize the following figure in your brain. If you do it, then you will see, how easy to calculate subnet mask. I again, request you to memorize the figure.

Cheat Chart of IP address and Subnet

Cheat Chart of IP address and Subnetting

Steps to calculate or to define subnet mask

Follow the following steps to quickly define the subnet mask:

Look at the IP address (you are given to calculate subnet) and determine the class and default subnet mask of that IP.

Find out the total no of subnets or subnetworks that are needed.

Find out the total no of bits required to satisfy that total no of subnets or subnetworks (comparing step 2).

Now, start to count out the total no of bits (from left to right) required to satisfy that subnet mask.

When you find the required bits, please stop then. These bits are
the bits to calculate the subnet mask and find out the value only those
bits. The value of those bits will be the subnet mask value.

The remaining no of right hand side bits are the host bits to define the hosts of that subnetwork.

Explanation with an Example

Let we have a IP address like 150.100.0.0. We want to sunbnetwork into 45 individual networks. To do it, we follow the above steps one by one.

Step 1: Look at the IP address (you are given to calculate subnet) and determine the class and default subnet mask of that IP.

The IP address is : 150.100.0.0. It's a class B address and the default subnet mask of this address will be 255.255.0.0.

Step 2: Find out the total no of subnets or subnetworks that are needed.

We are needed 45 individual subnets (as mentioned in the example).

Step 3: Find out the total no of bits required to satisfy that total no of subnets or subnetworks.

The binary form of 45 is 101101 and it requires only 6 bits to express 45. So, we are sure that we need 6 bits to calculate 45 subnets.

Generally, most people calculate in this fashion. But it's not a quick way to calculate. I will tell you the quickest way now. I already told you to memorize the cheat cart. I hope, you do. Now, always keep that chart in your brain. And calculate how long you have to count the bits. You see that when you raach the 6th bit, you can easily make 45 subnets. See the color of green digit. To calculate we only look the red field value. We need 45 value. Try to find the value which has included 45 from right to left. Go on and you will see that 1, 2, 4, 8, 16, 32 doesn't contain the value 45. Now 64 value. It contains 45. This is the first happening. So, our subnet will be just berfore the previous bits that means the red 32 value. We marked these bits by square. Count the total no of bits. I think you have done and you find 6 bits.

Try to find the no of bits required to support the subnets. Calculate from right to left until you reach the value which is high enough to support the desired sunbetwork value. Now, count the total no of bits to get the value. Here, we have value 45.

Step 4: Now, start to count out the total no of bits (from left to right) required to satisfy that subnet mask.

Count the total no of bits from left to right and put 1 which bits are required. For making subnet mask, all the subnet bits will be 1 and all the other bits i.e. the host bits will be 0.

Step 5: When you find the required bits, please stop then. These bits are
the bits to calculate the subnet mask and find out the value only those
bits. The value of those bits will be the subnet mask value.

From the figure, you can see that the chart allow you to determine the subnet mask value by simply identifying the number below the last bit which is used in the subnet mask.

Convert the binary bits (orange color) value to decimal value and you will see that it matches the value of chart.